The present invention is directed to braking devices for bicycles and, more particularly, to a bicycle hub brake which includes a mechanism for preventing overbraking.
A known bicycle hub brake is disclosed in Japanese Utility Model 2-35897. This bicycle hub brake includes a brake frame that supports a pair of brake shoes and a brake operation component. The brake shoes are disposed radially within a brake drum which rotates together with the bicycle wheel. In operation, the brake operation component pivots when a brake wire is pulled, and a cam surface on the brake operation component causes the brake shoes to move radially outwardly and press against the brake drum. The brake frame is connected to a support member through a spring so that, when a prescribed braking force is reached, the frictional contact between the brake shoe and the rotating brake drum causes the brake frame and brake shoes to move in the direction of the rotating drum against the force of the spring. This movement causes the brake operation device to move toward the brake wire housing which, in turn, creates slack in the brake wire to automatically reduce the frictional force between the brake shoe and the brake drum.
Operation of the prior art hub brake is shown graphically in FIG. 13. The inner wire operating stroke S is assigned to the horizontal axis, and the braking force B is assigned to the vertical axis. Until the brake wire operation stroke reaches a predetermined stroke SA, the braking force increases at a first angle of slope "a." After the operating stroke reaches the predetermined stroke SA and the braking force reaches the predetermined braking force BA determined by the spring, slackening of the brake wire causes the braking force to increase at a second angle of slope "b," where the slope "b" is smaller than the slope "a."
Some hub brakes use grease to control the frictional force between the brake shoe and the brake drum. When the grease is depleted, the frictional braking force increases excessively, and overbraking occurs. Even in brakes which are intended to operate without grease, rusting and dust penetration cause the frictional braking force to increase excessively, and overbraking again occurs. Fortunately, the known brake prevents such overbraking to a limited extent. For example, as indicated in FIG. 13, under normal conditions in which grease depletion or rusting has not occurred, the braking force is indicated by braking force line B2. When abnormal conditions prevail, the braking force is indicated by braking force line B1. The maximum braking force BX occurs in conventional hub brakes which do not have the compensation mechanism described above. In those hub brakes the variation of the braking force is shown by braking force line B3. Thus, the maximum braking force created under abnormal conditions in a hub brake of the type described above is braking force BY, located at the top of braking force line B1, and this maximum braking force BY is lower than the maximum braking force BX which occurs in conventional hub brakes.
Unfortunately, a hub brake which employs the compensation mechanism described above tends to be relatively large, since the brake frame must be constructed to allow some movement, and some accommodation must be made to couple a free end of the brake frame to a spring. Thus, a more compact hub brake which prevents overbraking is desirable.